Image reading apparatus are so adapted that the density of an original as divided into minute areas is read by an image sensor from area to area and then subjected to data processing including tone reproduction to obtain digital data of an image of the original, which is sent to an output device such as a printer.
In reading halftone images with the image reading apparatus, a periodic density pattern, i.e. a so-called moire pattern, is likely to occur when the relation between the dot pitch, the reading density and further the periodic characteristics of tone reproduction involves a delicate phase difference. The moire pattern renders printed-out images less legible.
As an attempt to eliminate the moire pattern, it has been proposed to practice the dither process, as one mode of tone reproduction, using a dither matrix of an altered size or pattern. However, this proposal is infeasible if the occurrence of the moire pattern is attributable to the reading density pitch.